Field
This invention relates generally to a high power fiber laser amplifier having a broadband output and, more particularly, to a high power fiber laser amplifier having a broadband output, where the amplifier employs one or more diffraction gratings to correct for the angular dispersion induced by a diffractive optical element (DOE) that provides coherent beam combination (CBC).
Discussion
High power laser amplifiers have many applications, including industrial, commercial, military, etc. Designers of laser amplifiers are continuously investigating ways to increase the power of the laser amplifier for these and other applications. One known type of laser amplifier is a fiber laser amplifier that employs a doped fiber that receives a seed beam and a pump beam to amplify the seed beam and generate the laser beam, where the fiber has an active core diameter of about 10-20 μm or larger.
Improvements in fiber laser amplifier designs have increased the output power of the fiber to approach its practical power and beam quality limit. To further increase the output power of a fiber amplifier some fiber laser systems employ multiple fiber laser amplifiers that combine the amplified beams in some fashion to generate higher powers. A design challenge for fiber laser amplifier systems of this type is to combine the beams from a plurality of fiber amplifiers in a manner so that the beams provide a single beam output having a uniform phase over the beam diameter such that the beam can be focused to a small focal spot. Focusing the combined beam to a small spot at a long distance (far-field) defines the quality of the beam.
In one known multiple fiber amplifier design, a master oscillator (MO) generates a seed beam that is split into a plurality of fiber seed beams each having a common wavelength, where each fiber beam is amplified. The amplified fiber seed beams are then collimated and directed to a diffractive optical element (DOE) that combines the coherent fiber beams into a single combined output beam. The DOE has a periodic structure formed into the element so that when the individual fiber beams each having a slightly different angular direction are redirected by the periodic structure all of the beams diffract from the DOE in the same direction. Each fiber beam is provided to a phase modulator that controls the phase of the beam so that the phase of all the fiber beams is maintained coherent. However, limitations on bandwidth and phasing errors limits the number of fiber beams that can be coherently combined, thus limiting the output power of the laser.
In another known multiple fiber amplifier design, a plurality of master oscillators (MOs) generate a plurality of fiber seed beams at a plurality of wavelengths, where each fiber beam is amplified. The amplified fiber seed beams are then collimated and directed to a diffraction grating, or other wavelength-selective element, that combines the different wavelength fiber beams into a single output beam. The diffraction grating has a periodic structure formed into the element so that when the individual fiber beams each having a slightly different wavelength and angular direction are redirected by the periodic structure all of the beams diffract from the diffraction grating in the same direction. However, limitations on bandwidth limit the number of fiber beams that can be wavelength-combined, thus limiting the output power of the laser.
To overcome these limitations and further increase the laser beam power, multiple master oscillators can be provided to generate seed beams at different wavelengths, where each of the individual wavelength seed beams is split into a number of fiber seed beams and where each group of fiber beams has the same wavelength and are mutually coherent. Each group of the coherent fiber seed beams at a respective wavelength are first coherently combined by a DOE, and then each group of coherently combined beams are directed to a spectral beam combining (SBC) grating at slightly different angles that diffracts the beams in the same direction as a single combined beam of multiple wavelengths. The SBC grating also includes a periodic structure for combining the beams at the different wavelengths.
It is often desirable that the output beam from a fiber laser amplifier be narrow linewidth, i.e., have a narrow frequency range, to improve beam quality. However, providing both high power and narrow linewidth has heretofore been challenging in the art because those requirements are typically incompatible with each other because higher power typically requires a wider beam linewidth. More particularly, the phenomenon of stimulated Brillouin scattering (SBS) i.e., back scattering of the beam as it propagates along the fiber amplifier, increases at narrower linewidths with small frequency ranges, which acts to reduce beam power. However, the wider the beam linewidth, the more difficult it is to coherently combine or spectrally combine beams from multiple fibers into a single beam through known beam combining techniques. Particularly, dispersion effects from an SBC grating require that the linewidth of the beams being amplified is narrow, where spectral dispersion causes the spectral components of the beam to be diffracted at different angles. In other words, for SBC, the spectral brightness of the seed beam directly limits the theoretical brightness of the combined beam output.
For coherent beam combining (CBC), the spectral brightness of the seed beam limits the beam combining efficiency as a result of imperfect matching of group delay and dispersion between amplifiers. Typically, the source spectral brightness is limited by SBS, and the seed beam source to the fiber amplifiers must be frequency-modulated to reduce the peak SBS gain and achieve the desired output power. The FM spectral broadening limits the attainable spectral brightness from a single fiber amplifier, thus limiting the system output.
Current high power fiber laser amplifiers typically operate with a center seed beam frequency of about 1 micron. In order to suppress SBS and other non-linearities in fiber amplifiers, the seed beam needs to be spectrally broadened. Current state of the art amplifiers providing 1-3 kW power levels typically need to be seeded with tens of gigahertz of optical bandwidth to properly operate. Future high power amplifiers may even need to be seeded with hundreds of gigahertzs of optical bandwidth to suppress non-linearities. These broad bandwidths can impact combining efficiency when using a DOE as a result of the dispersive nature of the DOE, where the DOE imparts a different angular dispersion to each of the input beams resulting in non-ideal overlap in the far-field and reducing combining efficiency. In other words, those beams that impinge the DOE at higher orders and are diffracted into the common output beam from the DOE have a certain dispersion based on their order that causes the beam to expand in the common beam differently than the other input beams impinging the DOE.
The impact on combining efficiency becomes significant when the angular dispersion is greater than 10% of the natural diffraction of the input beams to the DOE. The impact on combining efficiency can be mitigated by either decreasing the angular dispersion or increasing the natural divergence by decreasing the spot size on the DOE. However, the former increases the size of the beam combiner and the latter increases the beam intensity on the DOE, neither of which is desirable in typical high power fiber amplifier systems.